This invention pertains to a method and apparatus for inspecting substrates used during the manufacture of magnetic disks.
Magnetic disks are typically manufactured by the following process:
1. An aluminum alloy substrate is electroless plated with NiP.
2. The plated substrate is polished.
3. The polished substrate is then textured, either mechanically or using a laser.
4 An underlayer (e.g. Cr or NiP), a magnetic alloy (typically a Co alloy) and a protective overcoat (typically carbon, hydrogenated carbon, or zirconia) are then sputtered, in that order, onto the substrate.
5. A lubricant is then applied to the protective overcoat.
The layers formed on magnetic disks (e.g. the underlayer, magnetic layer and overcoat) are extremely thin, e.g. on the scale of several tens of nanometers. It is very important that there be no or few large defects in the substrate prior to sputtering.
It is known in the art to use laser scanning systems to inspect magnetic disk substrates prior to sputtering. Examples of such systems include the PMT Pit Detector, the Diskan 6000, Diskan 9000 and Diskan 9001 systems manufactured by QC Optics of Burlington, Mass. Other prior art systems are discussed in U.S. Pat. Nos. 4,794,264; 4,794,265; and 5,389,794, each assigned to QC Optics.
FIG. 1 schematically illustrates a QC Optics Diskan 9001 apparatus 10 for detecting defects in a substrate, such as a substrate 12. Referring to FIG. 1, apparatus 10 comprises HeNe lasers 14a, 14b for generating laser beams 16a, 16b respectively. Laser beam 16a is used to scan across and inspect one side of substrate 12, while laser beam 16b is used to scan across and inspect the other side of substrate 12. (Substrate 12 is typically rotated by a motor during this inspection, and laser beams 16a, 16b typically scan in the radial direction of the substrate.)
Laser beam 16a passes through a polarizer 18a, xc2xc waveplate 20a, and a shutter 22a, reflects off a mirror 23a, passes through a lens 24a, a beam splitter 25a, and a lens 26a and reflects off of mirror 28a. Mirror 28a deflects laser beam 16a downward to substrate 12. Substrate 12 reflects laser beam 16a upwardly and back to mirror 28a, through lens 26a and back to beam splitter 25a. Beam splitter 25a deflects laser beam 16b to a photomultiplier tube 30a. Of importance, if laser beam 16a strikes a defect in substrate 12 (either a pit or a bump), that defect will reflect laser beam 16a at an angle. The fact that laser beam 16a is reflected at an angle is detected by photomultiplier tube 30a. In this way, apparatus 10 can use laser beam 16a to determine whether there are pits or bumps in substrate 12.
The manner in which a defect deflects a laser beam can best be understood by comparing FIGS. 2A and 2B. In FIG. 2A, laser beam 16a strikes a portion of substrate 12 where defect 32 deflects laser beam 16a at an angle xcex8. In contrast, in FIG. 2B, laser beam 16b strikes a portion of substrate 12 where there are no defects. Thus, in FIG. 2B, laser beam 16a reflects straight back, and not at an angle. As mentioned above, photomultiplier tube 30a detects whether or not laser beam 16a is reflected at an angle by a defect on substrate 12.
Referring back to FIG. 1, portions of laser beam 16a are also reflected past mirror 28a, pass through spacial filter 34a and lens 36a, and strike photomultiplier tube 38a. (Spacial filter 34a filters out light scattering caused by the texture pattern that is formed on substrate 12.) Of importance, photomultiplier tube 38a determines whether light is scattered by defects or contamination on substrate 12 at a wide angle.
The optical path for laser beam 16b is similar to the optical path of laser beam 16a, and will not be described in detail, except to note that it includes two mirrors 28bxe2x80x2 and 28bxe2x80x3 instead of single mirror 28a. 
FIG. 3 is a block diagram of the circuitry coupled to photomultiplier tubes 30a, 30b, 38a and 38b. As can be seen, each of photomultiplier tubes 30a, 30b, 38a and 38b is coupled to four comparators 42a-42d, 44a-44d, 46a-46d and 48a-48d, respectively. Each of comparators 42a-42d compares the output signal OS30a of photomultiplier tube 30a with an associated reference voltage RV42a-RV42d, and provides a binary output signal BOS42a-BOS42d in response thereto. Binary output signals BOS42a-BOS42d are stored in associated latches 52a-52d, the contents of which are loaded into a memory which can then be accessed by a central processing unit CPU (not shown). Comparators 44-48 similarly compare the output signals from photomultiplier tubes 30b, 38a and 38b to reference voltage signals RV, and generate binary output signals BOS in response thereto. These binary output signals are stored in latches 54-58, the contents of which can be accessed by central processing unit CPU to determine the size and character of a defect detected by the apparatus.
While apparatus 10 can detect some defects, it would be desirable to provide improved means for detecting such defects with greater sensitivity and accuracy.
A method for inspecting a substrate in accordance with our invention comprises the step of providing a laser beam that strikes and reflects off the substrate and then strikes a bi-cell photodetector. In one embodiment, the photodetector is a photodiode. The cells of the photodetector are coupled to circuitry that generates a signal equal to (Lxe2x88x92R), where L is the strength of the signal provided by one cell of the photodetector, and R is the strength of the signal provided by the other cell of the photodetector. The signal Lxe2x88x92R corresponds to the difference between the amount of light striking one cell of the photodetector and the amount of light striking the other cell, which in turn depends on the extent to which the laser beam is deflected by a defect. A signal equal to L+R is also developed. Signal L+R is used to xe2x80x9cnormalizexe2x80x9d signal Lxe2x88x92R. In other words, signal L+R is used to compensate for sources of common mode noise, e.g. fluctuations in the intensity of the laser, variations in substrate reflectivity, etc. From these two signals, a signal proportional or equal to (Lxe2x88x92R)/(L+R) is developed. Signal (Lxe2x88x92R)/(L+R) is compared to a set of threshold circuits to determine the size of the defect detected.
In one embodiment, the bi-cell photodetector contains two photodiodes that are biased with a bias voltage so that the photodiodes exhibit reduced capacitance. Because of this, the circuit employing the bi-cell photodetector exhibits enhanced bandwidth, thereby improving the speed at which the substrate can be inspected.
We have found that one embodiment of apparatus in accordance with our invention is more sensitive to defects than the apparatus of FIG. 3. For example, the apparatus of FIG. 3 was capable of detecting defects having a wall slope of about 0.05xc2x0 or greater. One embodiment of our invention can detect defects having a wall slope less than 0.02xc2x0, and in one embodiment, defects having a wall slope as low as 0.005xc2x0. (A defect wall slope of 0.005xc2x0 typically represents the lower limit of presently feasible substrate manufacturing processes. If one could manufacture a flatter substrate, we believe the apparatus of the present invention could detect defects having wall slopes as low as 0.003xc2x0.)